1. Field of the Invention
The present invention relates to a displacement amount measuring device which measures the displacement amount of a movable body by using a displacement sensor which outputs a sensor signal corresponding to the displacement amount of the movable body, and in particular, relates to a technique for calibrating the displacement amount measuring device in accordance with changes in the characteristic of the displacement sensor.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2000-258109 discloses a displacement amount measuring device which measures the displacement amount of a movable element of a solenoid valve, by using a rod-shaped magnet which is interlocked with the movable element of the solenoid valve, and a magnetic sensing device (e.g., a Hall IC) which is held within a case assembled to the solenoid valve. Japanese Patent Application Laid-Open No. 2000-258109 also discloses that the neutral position is calibrated by learning control only one time after completion of assembly of the case and the solenoid valve, and that sufficient accuracy of measurement can be ensured merely by correcting the assembly error by this one-time calibration.
Japanese Patent Application Laid-Open No. S58-193403 discloses an angle detecting device in which, by using an angle sensor which is interlocked with rotation of a rotating body and emits a pulse signal each time the rotating body rotates by a unit angle and generates a reference pulse signal at a predetermined reference position, the angle value from the reference position is computed by counting the pulse signals inputted from the angle sensor, and the angle value is stored. This angle detecting device corrects the error of the angle value by resetting the stored angle value to zero when the reference pulse signal is inputted. However, if the angle value at the time when the reference pulse signal is inputted is outside of a range between a predetermined upper limit value and lower limit value, which can be considered to be reference positions, the angle value is not reset. In this way, erroneous resetting caused by high-frequency noise being mistakenly considered as the reference pulse signal, is prevented.
Japanese Patent Application Laid-Open No. 2002-273642 discloses the following in order to correct the feeding/driving of a ball screw of machine tool. Before machining, the ball screw is moved to the stroke end, and the coordinate value of the stroke end is detected. After machining, the ball screw is again moved to the stroke end, and the coordinate value of the stroke end is detected. On the basis of the difference in the coordinate values of the stroke end before machining and after machining (the amount of elongation of the main shaft due to the thermal expansion), the driving amount of the ball screw is corrected.
In the displacement amount measuring device disclosed in Japanese Patent Application Laid-Open No. 2000-258109, initial stage calibration of the neutral position of the movable body is carried out only one time at the time when the device is assembled, and no calibration whatsoever is carried out thereafter. However, the characteristic of the portion that first senses the displacement amount of the object, such as a magnet or a Hall IC or the like (a displacement sensor in the present specification), gradually changes over the long period of time of use of the displacement amount measuring device. For example, there are cases in which the magnetic force of the magnet gradually decreases or the position of the magnet shifts, or the circuit characteristics, such as the gain or the like, of the Hall IC also change. Due to such characteristics of the displacement sensor changing over time, the results of measurement of the displacement amount measuring device gradually include a large error.
In the angle measuring device disclosed in Japanese Patent Application Laid-Open No. S58-193403, the error in the angle measurement value, which is due to an error in counting the pulse signals, can be eliminated by resetting, to zero, the measurement value at the time when the rotating body is positioned at the reference position. However, the correction method, which is employed in Japanese Patent Application Laid-Open No. S58-193403 and in which the measurement value at the reference position is reset to zero, cannot eliminate errors in the measured value of the displacement amount, which errors are due to changes over time in the characteristic of the displacement sensor in a device which determines the displacement amount from a sensor signal value outputted from the Hall IC of a displacement sensor, such as the displacement amount measuring device disclosed in Japanese Patent Application Laid-Open No. 2000-258109. Namely, if the characteristic of the displacement sensor changes, because the non-linear relationship between the displacement amount and the sensor signal value changes over the entire displacement range, the errors in the results of measuring the displacement amount cannot be corrected over the entire displacement range, even if only the measured value of the one point which is the reference position is reset to zero.
In the ball screw feeding/driving device disclosed in Japanese Patent Application Laid-Open No. 2002-273642, the error in the ball feed amount, which is caused by thermal expansion of the main shaft during machining, can be corrected on the basis of the difference in the coordinate values of the stroke end before and after machining. However, in a device, such as the displacement amount measuring device disclosed in Japanese Patent Application Laid-Open No. 2000-258109, in which the non-linear relationship between the displacement amount and the sensor signal value changes over the entire displacement range due to changes in the characteristic of the displacement sensor, it is not possible to correct errors in the results of measurement of the displacement amount over the entire displacement range, merely from the difference in the coordinate values at the one point which is the stroke end.